1. Technical Field
The present invention relates to ion mobility spectrometers used for gas and vapour detection, and more particularly to ion mobility spectrometer systems in which the system is xe2x80x9cdopedxe2x80x9d, or has added to it, a low concentration of a trace reagent vapour or vapours (the xe2x80x9cdopantxe2x80x9d) e.g. to improve the sensitivity of the system to gases or vapours of interest, or to improve the rejection of interfacing materials (i.e. those which may otherwise give rise to a response interfering with detection of gases and vapours of interest).
2. Background of the Invention
The use of dopants in Ion Mobility Spectrometer (IMS) systems is well known and the principles involved have been described in the literature, for example, in the introduction to EP-A-219602.
Dopant sources commonly consist of a sealed container with a permeation capability containing the chosen dopant material, with the container incorporated in the circulating system of the IMS detector, comprising the ion mobility cell, means such as a sieve pack for drying and cleaning the recirculating gases in the system, the dopant source, and a pump, into which samples of gases or vapours of interest, usually air-borne, are drawn for analysis.
In accordance with one aspect, the present invention consists in an ion mobility spectrometer system in which the dopant material is physically combined with molecular sieve material, whereby the need for a physically separate dopant source for the system is obviated.
In accordance with another aspect, the invention consists in a combined dopant source and molecular sieve, for use, for example, with an ion mobility spectrometer.
In accordance with a still further aspect the invention consists in a method of physically or chemically combining a molecular sieve material with a dopant material to provide a combined dopant and molecular sieve for use with an ion mobility spectrometer.
The invention provides a number of advantages over current practice in which separate dopant sources and molecular sieve packs are employed.
For example, in hand-held or body-worn IMS equipment, space within the equipment housing is at a premium, and use of a combined dopant source/molecular sieve reduces the space requirement of the circulating system.
Additionally, and especially in small instruments, there is a need to regularly change the molecular sieve. By providing a combined sieve and dopant source in a single pack, a dopant change, desirable for maintaining a consistent level of dopant within the circulating system, is provided at the same time by a single action.
Current practice requires dopant sources associated with IMS circulating systems to be heated when the instrument is operated at low temperatures. With a combined sieve and dopant source it has unexpectedly been found possible to maintain dopant levels within the circulating system at low temperatures without the need for heating, thereby further simplifying and reducing the space and power requirements of the instrument.
It has been found that combination of the dopant and the sieve material provides the required level of sensitisation of the IMS instrument to samples of interest and/or rejection of interferents.
Use of the combined dopant and sieve material within the circulating system of an IMS instrument, has also been found to improve dopant capability over a range of ambient temperatures from xe2x88x9230 C. to +50 C., even without heating.
Although it might be expected that applying dopant to a sieve would increase water takeup, and hence reduce the useful life of the product, the applicants have found in practice that that is not in fact the case.